There is a strong demand for systems or sensors that can detect the presence of hazardous materials, such as explosive materials, particularly those with high sensitivity and specificity, as well as the capability for standoff detection. Primary, secondary and tertiary explosives make up the three classes of high explosive materials, each having decreasing sensitivity to shock, friction, and heat. Peroxide-based explosives (e.g., acetone peroxides) are one of the main constituents of primary explosives, while nitro-based explosives make up the majority of secondary explosives (e.g., trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), pentrite (PETN), while tertiary explosives include ammonium nitrate/fuel oil (e.g., ANFO).
Direct detection of explosives using native fluorescence of the target substance is difficult because the fluorescence spectra are typically broad and structureless/featureless. Selective photofragments from photodissociation of explosive materials have strong fluorescence that produces structured or feature-evident spectra. Nitric oxide (NO) is a characteristic photofragment of nitro-based explosive materials when the explosive material is irradiated with ultraviolet (UV) light. Specifically, absorption by NO via its various A-X (v′,v″) bands, e.g. (0,0), (1,1), (2,2), and (0,2) is known to have transitions near 226, 224, 222, and 248 nm, resulting in discrete characteristic laser-induced fluorescence (LIF) emissions.
Typically LIF measurements are performed using 226 nm laser excitation. However, 226 nm excitation is plagued by a background of natural NO in the air, making it difficult to detect low vapor pressure compounds such as certain explosives (e.g., 2,4,6 trinitrotoluene (TNT)), which may have a concentration from around 1 to 10 parts per billion (PPB) in typical samples. Most nitro-based explosives have a low NO vapor pressure that combined with the low concentration in the sample generally results in a low signal level for the LIF signal emanating from the sample. Another known option is to use 236 nm laser excitation, and to detect fluorescent emission peaks at 226 nm or 247 nm, which represent the NO peaks having the highest and second highest relative signal strengths in the conventional spectral range of interest for PF-LIF for nitro-based explosives, being from 200 to about 250 nm.
Problems with known PF-LIF systems for NO-based explosives include detectability problems and low throughput due to low emission signal levels at the photodetector due to the low signal level of interest emanating from the sample combined with the signal loss through the spectral filter (low % transmission of the emission signal transmitted) before detection at the photodetector. A highly attenuating spectral filter is required in conventional PF-LIF systems due to the need to selectively detect the emission peak of interest at 226 nm or 247 nm while rejecting the closely spaced laser peak in the wavelength range about 225 nm to 250 nm. Moreover, interference from atmospheric O2 fluorescence can confuse signal analysis both within the wavelength range from about 225 nm to 250 nm, and outside this wavelength range.